1. Field of the Invention
The present invention relates to a uniaxial drive unit and a surface shape measuring apparatus using the unit. More particularly, it relates to a uniaxial drive unit using a linear motor in a driving section and a surface shape measuring apparatus using the unit.
2. Description of the Related Art
As a linear motor capable of being used for a uniaxial drive unit, linear motors having various configurations have conventionally been known. Among these, a linear motor having a fixed part, which is a rod-shaped magnet, and a moving part, which is a ring-shaped member fitted on the fixed part, having a coil member, and being capable of moving linearly along the fixed part, has features of less cogging, less unevenness of speed, and the like, and hence has been coming onto the market (for example, trade name: SHAFT MOTOR, manufactured by GMC HILLSTONE Co., Ltd.).
Also, as an improved technology for such a linear motor, a linear motor capable of operating steadily with high accuracy has been proposed (for example, see Japanese Patent Application Publication Nos. 8-331834 and 11-150973).
On the other hand, as a surface shape measuring apparatus, a surface roughness measuring apparatus, contour measuring apparatus, roundness measuring apparatus, three-dimensional coordinate measuring apparatus, and the like have conventionally been known.
In most of the surface shape measuring apparatuses, a contact type probe and an object under test are moved relatively while the probe is brought into contact with the surface of the object under test, by which the surface shape of object under test is measured.
The relative movement of the probe and the object under test is generally linear motion for the surface roughness measuring apparatus, contour measuring apparatus, and three-dimensional coordinate measuring apparatus, and is generally arcuate motion for the roundness measuring apparatus. Also, for the roundness measuring apparatus as well, the Z-axis movement (vertical movement) of probe is generally linear motion.
The driving for such linear motion is generally performed by a combination of a motor, gear, and screw or a combination of a motor, pulley, and wire (Japanese Patent Application Publication No. 2002-139317).
Specifically, in the surface roughness measuring apparatus, contour measuring apparatus, and roundness measuring apparatus, linear driving is performed by the combination of a motor, gear, and screw, and a contact type detector is moved linearly while being supported by a slide guide surface (or hydrostatic bearing surface) having high straightness accuracy, by which the surface roughness, contour, roundness, or the like of an object under test is measured. Also, in the three-dimensional coordinate measuring apparatus; linear driving is performed by the combination of a motor, pulley, and wire, and a contact type detector is moved linearly while being supported by a hydrostatic bearing surface having high straightness accuracy, by which the shape and dimensions of an object under test are measured.
In this case, generally, the linear driving of the surface roughness measuring apparatus and contour measuring apparatus is in the X-axis direction, that of the roundness measuring apparatus is in the R-axis and Z-axis directions, and that of the three-dimensional coordinate measuring apparatus is in the X-axis, Y-axis, and Z-axis directions.
However, when the above-described conventional linear motor is applied to the uniaxial drive unit, the linear motor generates heat, so that a problem is pointed out in that a dimensional error of the whole of unit occurs due to the heat.
Also, in the case where the above-described conventional linear motor is applied to the uniaxial drive unit and the unit is used in a state in which the driving thrust is varied by the gravity of the moving part, a problem is also pointed out in that the variation in the driving thrust is difficult to restrain.
FIGS. 13 to 15 are schematic views for illustrating this phenomenon. FIG. 13 schematically shows a cross section of a linear motor 1. A moving part 4, which is a ring-shaped member having a coil member, is fitted on a fixed part 2, which is a rod-shaped magnet in which the N poles and the S poles are arranged alternately in a linear form. Due to the interaction between the magnetic flux of the fixed part 2 and the electric current flowing in the coil member of the moving part 4, the moving part 4 moves linearly along the fixed part 2 in accordance with Fleming's left-hand rule. The coil member of the moving part 4 is supplied with an electric current by a driving circuit, not shown.
In this case, if friction is present between the fixed part 2 and the moving part 4, states as shown in FIGS. 14 and 15 are established when the linear motor 1 is not driven by energization. FIG. 14 shows a state in which the linear motor 1 is arranged horizontally, and FIG. 15 shows a state in which the linear motor 1 is arranged at an inclination angle Θ with respect to the horizontal surface. The friction coefficient between the fixed part 2 and the moving part 4 is taken as μ.
In FIG. 14, there is no relative movement of the fixed part 2 and the moving part 4 in the state in which the moving part 4 is not energized. In order to move the moving part 4 to the right or the left, a driving force F=μM is required to overcome the frictional force caused by the gravity M of the moving part 4.
On the other hand, in FIG. 15, a component force M·Sin Θ due to the gravity of the moving part 4 is applied in the linear movement direction along the fixed part 2. Also, a component force M·Cos Θ due to the gravity of the moving part 4 is applied in the direction perpendicular to the linear movement direction along the fixed part 2.
In order to move the moving part 4 to the left by overcoming the component force due to the gravity of the moving part 4 and the frictional force in this state, a driving force of F=M·Sin Θ+μM·Cos Θ is required as shown in FIG. 15A. On the other hand, in order to move the moving part 4 to the right, a driving force of F=−M·Sin Θ+μM·Cos Θ is required as shown in FIG. 15B. Thus, in the conventional linear motor of this type, the driving thrust is varied by the gravity of the moving part 4.
Also, if the angle Θ is large in the state as shown in FIG. 15, the moving part 4 moves to the right (drops) due to the gravity of the moving part 4. In order to restrain this phenomenon and to keep the moving part 4 at the present position, it is necessary to provide a driving force in the left direction by energizing the moving part 4 to achieve a balance. However, if the state in which the moving part 4 is energized is maintained, heat is generated in the moving part 4, which presents a problem in that a dimensional error of the whole of unit occurs due to the heat.